Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of the data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, optical disk cartridges, floppy diskettes, or floptical diskettes. Typically, magnetic tape is the most economical means of storing or archiving the data. Storage technology is continually pushed to increase storage capacity and storage reliability. Improvement in data storage densities in magnetic storage media, for example, has resulted from improved medium materials, improved error correction techniques and decreased areal bit sizes. The data capacity of half-inch magnetic tape, for example, is now measured in hundreds of gigabytes on 512 or more data tracks.
The improvement in magnetic medium data storage capacity arises in large part from improvements in the magnetic head assembly used for reading and writing data on the magnetic storage medium. A major improvement in transducer technology arrived with the magnetoresistive (MR) sensor originally developed by the IBM® Corporation. Later sensors using the GMR effect were developed. AMR and GMR sensors transduce magnetic field changes to resistance changes, which are processed to provide digital signals. Data storage density can be increased because AMR and GMR sensors offer signal levels higher than those available from conventional inductive read heads for a given read sensor width and so enable smaller reader widths and thus more tracks per inch. Moreover, the sensor output signal depends only on the instantaneous magnetic field intensity in the storage medium and is independent of the magnetic field time-rate-of-change arising from relative sensor/medium velocity. In operation the magnetic storage medium, such as tape or a magnetic disk surface, is passed over the magnetic read/write (R/W) head assembly for reading data therefrom and writing data thereto.
In any of the above-mentioned types of heads, e.g., AMR, GMR, MTJ, etc., forces are present that can produce adverse effects on the head. For example, in a tape-based data storage system, the tape acquires a charge as it moves through the tape path and over the head. In the head, sensor element potentials are generally established by the circuits that provide the sensing currents passing therethrough. The sensor shield potentials may or may not match the potentials on the associated sensor elements. Further, the sensor shields and other parts of the head may become charged by interaction with the tape. Thus, as can be appreciated, multiple and varying voltages may be found not only between the tape and the head, but also between various components in the head, e.g., substrate, shields and sensor element. As will soon become apparent, these voltages are believed to be at least partly responsible for adverse tribological interactions on the head.
Compounding the problem is the electric field distribution in the head ceramic. The wafer material used for magnetic recording heads (commonly know as AlTiC), is a ceramic composite material comprising a matrix of insulative aluminum oxide (alumina, Al2O3) plus an irregular but interconnected network of electrically conductive titanium carbide (TiC). The uneven distribution of TiC creates an irregular electric field at the interlace with the typically overlying insulator. The electric fields at grain edges can be several fold higher than the average field. These high fields are associated with adverse tribological effects, including electrostatic debris deposition and pitting of the head insulator due to electric discharges. These high fields also promote electrical discharge into the tape, thereby charging the tape.
Due to the varying voltages and irregular electric fields between the various parts of a head as well as between head and tape, magnetic heads tend to suffer from adverse tribological interactions, which include electrical discharge, tape changes, head erosion, debris buildup, chemical conversion, head sensor shorting, etc. In both piggyback and interleaved heads, tribological effects are believed to be aggravated by excessive substrate and/or media voltages. When the substrate is electrically floating, relative motion between the head and recording medium may produce substrate and media voltage swings on the order of several 10 s of volts. Such voltages are strongly implicated in unfavorable tribological processes such as electrochemical reactions, electrostatic accumulation of debris, and even certain types of wear.
In shielded MR heads, the lower reader shield of each reader is in close proximity to the substrate, separated therefrom by a thin insulator, e.g., on the order of 0.5 to 5 micron thick. The voltage differences between the lower reader shields and substrate is problematic due to their close proximity, and is compounded by the potentially large localized electric fields created by a conductive substrate, e.g., of AlTiC. Such electric fields are implicated in aggravated accumulation of conductive materials that can actually short the MR sensor to its shields and in turn to the substrate.
Consider the following example. Suppose reader shield S2 and the substrate are separated by a thin insulator. Suppose shield S2 (positioned towards the substrate) is at 1.5 V, and the substrate is at 6.5 V. The difference is 5 V. If the space between them is 0.5 microns, the electric field (gradient) is 10 V per micron, a very large value. For comparison, sparking in air, for example as observed on clothing, occurs from a gradient of approximately 1 V per micron. In the example presented, the gradient is ten times larger. Other unusual effects have been observed, including formation of solid water at room temperature in the presence of large electric fields. Furthermore, the irregular conductive grain structure of AlTiC further concentrates the fields, which can be several times higher than in this example.
Several solutions have been contemplated, but each of these has drawbacks. These solutions include biasing the substrate to match the potential of the shields. However, this requires that additional biasing circuitry be coupled to the substrate, thus contributing to the overall complexity of the system. Grounding the shields in a multi-sensor head is generally impractical. Even if the shields are grounded, adverse tribological effects may occur depending on the tape electrical and mechanical characteristics and other aspects of the tape path, such as ground or floating of guides. Further, any shorting between the sensor and a grounded shield could result in MR signal reduction or noise, or even no signal.
As alluded to above, another problem encountered is that the readers are susceptible to shield-shorting which may occur in combination with substrate shorting, as a result of running magnetic recording tape having insufficient lubricity across the head at very low humidity, which in turn is found to produce accumulations of conductive material on the MR elements, shields and substrate. Shorting is a well-known cause of reading errors. Proposed solutions, such as prerecessing and/or insulating heads, providing sensor-piggybacked fences and running ionization fans, require changes in head processing, design or implementation, respectively, and so are far more involved than the present invention. For instance, forcibly recessing the sensor so that its components do not develop the conductive accumulation is difficult to manufacture, and also generally produces undesirable magnetic signal spacing loss for the data readers, which must read much higher frequencies than the servo readers.
There is accordingly a clearly-felt need in the art for a magnetic device with reduced susceptibility to shorting and/or improved tribological characteristics. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.